1. Field of the Invention
This invention relates generally to energy harvesting systems. It relates particularly to piezoelectric transducers for the concersion of mechanical energy to electrical energy.
2. Description of Related Art
Mobile computing, communication, and sensing devices have grown significantly in the past decade, as have various sensor networks, now used in a broad spectrum of applications. The demand has been increasing for a lightweight power supply having low volume, a high energy density, and a long lifetime. For a large integrated system such as an aircraft or ship, central electric power can be supplied to sensors and control units at each component; however, wiring from the central power source has been a big burden for these systems, and there have been concerns about system maintenance and reliability. A distributed system such as an environmental monitoring system requires thousands of various sensors, signal processing units, and local power supplies. Electronics consumers have long desired compact and long lifetime power supplies for small systems such as body wear (cell phone, PALM, DVD player, etc.)
Research and development efforts to address these problems can be divided into two main areas: batteries and energy harvesting. Advances in various batteries have indeed enhanced the capabilities of many devices; however, the operating lifetime of a battery-powered system limits system deployment time.
Energy harvesting is, then, an alternative approach to reclaim energy from available sources in the system's environment and to convert it into electrical energy to power a control system. Several exploited ambient sources include solar, heat/thermal, electromagnetic, RF, and mechanical vibration sources. Of these, mechanical vibration sources are varied in their origin, and the amount of power to be harvested therefrom is abundant. Electromagnetic and piezoelectric transducers are two major devices for energy harvesting in vibration environments. Piezoelectric transduction is well known to have a higher energy density and less environmental interference than its electromagnetic counterpart.
Progress has been made in piezoelectric transducers for the conversion of mechanical energy (mechanical shock, strain, stress, vibrations, and acoustic waves) to electric energy. Piezoelectric transduction mechanisms studied so far include piezoelectric stack transducers, unimorph/bimorph transducers, and flextensional transducers. Of these, flextensional transducers have shown a higher energy conversion efficiency in comparison to the others because strain/stress amplification mechanisms are employed by configurational design. However, when pure compliant amplification mechanisms are used, energy waste still exists, indicating that more efficient electromechanical transduction mechanisms should be developed. On the other hand, piezoelectric materials play an important role in electromechanical transducers for energy harvesting. Piezo-ceramics, electroactive polymers(EAPs), and single crystal piezoelectric materials are the primary available materials at present. Of these, the EAP stack transducers previously reported show significantly high power output; however, net power output is small due to the consumption of the input DC-Bias field to convert the EAP from electric to piezoelectric status.
Piezoelectric transducers with a very high energy conversion efficiency are required in broad variety of advanced technologies, but they are yet unavailable in the art.